This invention relates to fully-filled telecommunication cables and to compositions for use as a filling medium in them. These cables comprise a multiplicity of conductors each insulated with cellular polyethylene or polypropylene and enclosed in a sheath, the interstices between the insulated conductors, and between them and the sheath, being filled with a waterproof filling medium.
Such cables usually have insulation of cellular polyethylene or cellular polypropylene, which have surface energies of about 32.times.10.sup.-3 J/m.sup.2 and 30.times.10.sup.-3 J/m.sup.2 respectively. If the filling medium is to be effective it must wet the surface of the insulation, and this implies that it must have a free-surface energy lower than that of the insulation. Hydrocarbon oils gelled with waxes or other suitable gelling agents, and especially petroleum jelly, with a surface energy of about 28.times.10.sup.-3 J/m.sup.2, are amongst the few non-volatile materials that satisfy this requirement as well as the other important requirements of low permittivity and low dielectric loss. Petroleum jelly has other desirable properties and has been found entirely satisfactory for cables operating at temperatures of up to about 50.degree. C.
In some cases, however, it is desirable to use fullyfilled cables at temperatures up to about 80.degree. C.--for example when they are associated with and run alongside large power cables--and is this case two difficulties arise: first petroleum jelly (which is largely molten at these temperatures) shows a tendency to fill cells in the insulation to an extent that may be appreciable in a few months; and second the viscosity of the medium decreases to the point at which it may flow along the interstices under the hydrostatic pressures that may occur in an installed cable.
Attempts have been made to overcome these problems by increasing the effective viscosity of the gelled oil by incorporating into it soluble high polymers or mineral powders that impart thixotropic character. These measures have been reasonably successful in preventing flow of the gelled oil, but have had only a marginal effect on the temperature at which cell filling is observed.